The disclosures herein relate generally to solder ball electronic interconnections and more particularly to a via plug adapter for strengthening a solder ball connection in a beveled via.
Vertical interconnects between circuit layers is well known. U.S. Pat. No. 3,541,222 discloses a connector screen for interconnecting aligned electrodes of adjacent circuit boards or modules. The connector screen comprises a matrix of spaced conductive connector elements embedded in a supporting non-conducting material with the conductive connector elements protruding from both sides thereof. The size and spacing of the connector elements are chosen so that the connector screen can be disposed between the circuit boards or modules to provide the required interconnections between the electrodes without requiring alignment of the connector screen with respect to the boards or modules. A preferred method of making the connector screen involves forming a conductive mold having a grid pattern of ridges in a non-conductive base. Conductive material is then cast between the ridges of the mold, following which selected portions of the mold are removed to form a web of non-conductive material supporting a matrix of spaced conducting elements protruding from both sides of the web.
U.S. Pat. No. 4,830,264 describes a method of forming solder terminals for a pinless module, preferably for a pinless metallized ceramic module. The method is comprised of the following steps: forming a substrate having a pattern of conductors formed onto its top surface and preformed via-holes extending from the top to bottom surface; applying a droplet of flux at at least one of the preformed via-hole openings of the bottom surface of the substrate to fill the via-holes with flux by capillarity and form a glob of flux at the bottom openings; applying a solder preform, i.e. solder balls on each glob of flux to which it will adhere, the volume of the preform being substantially equal to the inner volume of the via-hole plus the volume of the bump to be formed; heating to cause solder reflow of the solder preform to fill the via-hole and the inner volume of the eyelet with solder; and, cooling below the melting point of the solder so that the molten solder solidifies to form solder terminals at the via-hole locations while forming solder columns in the via-holes. The resultant pinless metallized ceramic module has connections between the I/O""s of the module interfacing with the next level of packaging (i.e., printed circuit boards), that consist of integral solder terminals. Each integral solder terminal comprises a column in the vias of the metallized ceramic substrate, a mound of solder at the top surface of the substrate and spherical solder bumps on the bottom level for making interconnections with the next level of packaging.
In U.S. Pat. No. 5,401,913, a multi-layer circuit board includes electrical interconnections between adjacent circuit board layers of the multi-layer board. A via hole is provided through a circuit board layer. The via hole is filled with a via metal. The via metal is plated with a low melting point metal. An adhesive film is deposited over the circuit board layer. Adjacent layers of the multi-layer circuit board are stacked and aligned together. The layers are laminated under heat and pressure. The low melting point metal provides an electrical interconnection between adjacent layers.
U.S. Pat. No. 5,491,303 discloses an interposer for connecting two or more printed circuit boards comprising a circuit-carrying substrate with two or more solder pads on each of two sides. Each of the solder pads are connected to an electrically conductive via in the substrate, providing electrical interconnection from one side to the other side. Each solder pad has a solder bump on it. A circuit assembly is made by soldering the solder bumps on one side of the interposer to corresponding solder pads on a printed circuit board. The solder bumps on the other side of the interposer are likewise soldered to the corresponding solder pads of a second printed circuit board.
U.S. Pat. No. 5,600,884 describes an electrical connecting member, one surface of which is connected to a connecting section of a first electrical circuit member and another surface of which is connected to a connecting section of a second electrical circuit member. The electrical connecting member includes a holding member formed of an electrically insulative member. The holding member has a plurality of recess holes. The connecting member also includes a plurality of electrically conductive members provided in the electrically insulative member, insulated from each other. One end of the electrically conductive members is exposed on one surface of the holding member to be connected to the connecting section of the first electrical circuit member. Another end of the electrically conductive members is exposed on another surface of the holding member to be connected to the connecting section of the second electrical circuit member.
U.S. Pat. No. 5,726,497 discloses a method of manufacture of a semiconductor device on a silicon semiconductor substrate which comprises formation of a first stress layer on the semiconductor substrate, formation of an interconnect layer over the first stress layer, formation of a second stress layer on the interconnect layer, formation of an inter-metal dielectric (IMD) layer over the second stress layer, patterning and etching a via opening through the inter-metal dielectric layer and the second stress layer, exposing a contact area on the surface of the metal interconnect layer, and heating the device at a temperature sufficient to squeeze the metal interconnect layer up into the via.
U.S. Pat. No. 5,757,078 discloses a semiconductor device including a semiconductor chip having electrode pads, a package composed of a plurality of insulating films and adhered to the semiconductor chip by an adhesive agent. The package includes wiring patterns interposed between the plurality of insulating films. The wiring patterns are selectively connected to the electrode pads at one end, and to the plurality of electrically conductive protrusions at the other end, by means of via-holes. The semiconductor device further includes a plurality of electrically conductive protrusions extending from the outermost wiring patterns by way of the via-holes provided in the outermost insulating film.
Japanese Application JP 10-41356 discloses a tape carrier that is used as the bonding medium when semiconductor elements are bonded to the outer part of a substrate board for a BGA application. An insulating film includes vias having straight or non-tapered walls. A conductive land is formed in the vias and solder balls have one side engaged with the lands inside of the vias. The remainder of each solder ball protrudes from the insulating film.
The use of flexible circuitry in IC packaging has been a growing trend for many years where the use of via connections through the flexible circuit dielectric have been employed in Tape Ball Grid Array (TBGA) IC packaging applications and recently, into Chip Scale Packaging (CSP) applications. In Ball Grid Array (BGA) applications, the via interconnection traditionally uses a solder ball reflowed first to connect to the flexible circuitry through the via, then second, reflowed onto the printed circuit board with conventional surface mount assembly practices.
This solder ball connection must make a reliable electronic interconnect from the flexible circuitry to the printed circuit board. This reliability is often directly related to the area of the solder connection to the flexible circuitry, as a common failure mode of this interconnection is the solder ball shearing through the solder material at the point of the minimum cross sectional area. Therefore, larger vias are desirable to increase the area in which shear stress is distributed to meet minimum solder ball interconnection reliability requirements.
Conversely, the demand for smaller electronic packages and higher input/output (I/O""s) requires increased routing density, including smaller via sizes to allow electronic traces to route between solder ball via areas. Smaller vias require smaller via capture pads, thus, allowing more space to route electronic traces between printed circuit board interconnection vias.
Traditionally, vias in the dielectric are made by punching, leaving a via through the dielectric with straight walls. Other methods include chemically dissolving the dielectric and laser drilling to expose the metal conductor of the flexible circuitry. Direct solder ball attachment to any of these via methods controls the solder ball interconnection reliability by means of the via size, such that, vias typically have to be larger than 0.200 mm in diameter to meet minimum reliability requirements for the electronic package.
Therefore, what is needed is an apparatus and a method for providing a strong and reliable solder ball connection to flexible circuitry with small diameter vias and via capture pads so as to permit more space in which to route more electronic traces.
One embodiment, accordingly, provides a strength enhanced solder ball connection to flexible circuitry with small diameter vias which improves the routability of the flexible circuit to address higher I/O and finer pitch flex based BGA packaging applications. To this end, a circuit comprises a substrate including a dielectric layer having a first surface and a second surface. A conductive layer is on the first surface. A beveled via is formed in the dielectric layer and has a first opening of a first width in the first surface, and a second opening of a second width in the second surface, greater than the first width. A conductive plug is formed in the via, connected to the conductive layer, and extends from adjacent the first opening toward the second opening. The plug terminates adjacent the second opening at a plug interface surface. A conductive solder ball is connected to the plug interface surface and extends to protrude from the second surface.
A principal advantage of this embodiment is that the via adapter plug enables a reliable solder ball connection to flexible circuitry with small (less than 0.200 mm diameter) vias. Using the via plug adapter concept, solder ball interconnection reliability does not have to be compromised to accommodate the routing requirements of high I/O, fine pitch flex based IC packaging applications. Using common design rules for flexible circuitry, a smaller via allows for a smaller via capture pad, thus, more space between via capture pads in which to route electronic traces.